Breather hole - Early 750GT 'piecrust' fuel cap?

[SZRacer]

Hi all,

Does anyone know if/how the 'piecrust' shaped fuel cap on the early GTs breathes?

I've never had a problem before, but yesterday, about 10 minutes after filling the tank up, my bike coughed and spluttered to a halt after some high speed riding, which cured itself after removing the fuel cap and refitting.

This leads me to believe it's either the lack of breathing, or the fuel had dislodged some dirt in the tank.

Anyone know if/where these caps are supposed to breathe?

 

[Haggis]

I cant answer your question, but have just had a breathing problem with an old BMW. I found that the breather was blocked, not by dirt, but what I think was fuel evaporation build up - forming a plug. Much like a blocked jet on a standing carburettor I suppose. Maybe soak your cap in carb cleaner for a while - it worked for me.

HTH, Paul

 

[Tippie]

Unless a fuel tank has it's own breather pipe it HAS to breathe through the fuel cap. There will be a chain of small holes leading to the atmosphere. They can get blocked.

 

[Paul Marx]

The fuel cap doesn't breathe.
There's a pipe in the tank itself which ends at the top of the tank near the cap and exits at the rear right of the tank into a tube which should lead behind the engine.
That tube is not an overflow, it is the breather. Make sure it isn't blocked, particularly if you've used a resin of some sort in the tank.

Paul

 

[SZRacer]

Thanks, Paul.

Funnily enough, I noticed the breather spigot on the rear bottom of the tank, after I had it painted. In fact I asked what it was on the Facebook group!

I connected some fuel tube to it (there was none on it before), and routed it out the back by the tool tray. I did poke a drill bit into it, but it hit a 'stop'. I assumed there might have been holes on the side of the intrnal tube, but maybe not! I felt nervous of simply drilling up into the tank, as wasn't sure of the internal arrangement.

Sounds like my breather may indeed be blocked, then. The tank hasn't been relined, and in fact internally is pretty good, but I guess it's possible the painters could have blocked it.

If I carefully drill up the spigot, should it come out into 'clean air'? How near to the top of the tank?

The fuel cap doesn't breathe.
There's a pipe in the tank itself which ends at the top of the tank near the cap and exits at the rear right of the tank into a tube which should lead behind the engine.
That tube is not an overflow, it is the breather. Make sure it isn't blocked, particularly if you've used a resin of some sort in the tank.

Paul

 

[piranha-bro2]

I must confess in advance that I'm not at all familiar with the GT remote breather tube setup, but unless the tube inside the tank is dead straight (highly unlikely) I would definitely NOT send a drill up there! Without knowing how it works, I'd be concerned you'll create a hole into the fuel cavity, that will then leak out the breather spigot.

Can you get some stiff welding wire or similar and run that up? However the system works, it's just a matter of ensuring it allows air to pass through ... locating its passage is the first step.

 

[Exmoor beast]

Just had a look at mine and it has a breather hole in cap and no tank spigot r/hand rear and no internal pipe! The hole lines up with the grooves on underside of cap below the gasket ring.

 

[Paul Marx]

Just had a look at mine and it has a breather hole in cap and no tank spigot r/hand rear and no internal pipe! The hole lines up with the grooves on underside of cap below the gasket ring.

I have a cap like that, another is drilled with a tiny hole right in the middle, probably an owner mod. It's not the case with my S or my 650 tank. Photo of the S below. That one was blocked with resin and one of the many reasons getting that machine roadworthy was a right pain. I have another 2 early GT tanks, but they aren't accessible without risking life and limb.

That breather/overflow pipe is very convoluted. By no means try to clear it with anything rigid. Start by giving it a blowjob to see if it's free and otherwise, use some wire to try and unblock it. I suspect that the tube may well be copper so fragile.

Paul

 

[Exmoor beast]

Hi all,

Does anyone know if/how the 'piecrust' shaped fuel cap on the early GTs breathes?

I've never had a problem before, but yesterday, about 10 minutes after filling the tank up, my bike coughed and spluttered to a halt after some high speed riding, which cured itself after removing the fuel cap and refitting.

This leads me to believe it's either the lack of breathing, or the fuel had dislodged some dirt in the tank.

Anyone know if/where these caps are supposed to breathe?

Check the taps- mine had a very reduced flow due to crud buildup in the internal passageways. I know removing the cap seemed to cure the problem but the time taken  to stop and remove cap will allow the carbs to fill, albeit with reduced flow, giving the same effect. At least you know it?s lack of fuel , got to be taps or breather. Your bike looks cool by the way.

 

[Paul Marx]

Check the taps- mine had a very reduced flow due to crud buildup in the internal passageways. I know removing the cap seemed to cure the problem but the time taken  to stop and remove cap will allow the carbs to fill, albeit with reduced flow, giving the same effect. At least you know it?s lack of fuel , got to be taps or breather. Your bike looks cool by the way.

No need to stop, unscrew the cap on the move.
No time to waste.
Paul

 

[CLEMTOG]

as Paul says, first give it a blow job, but not from the top, reverse blow it up the arse (ie the outlet), push a short piece of hose over the outlet and then stick the airgun nozzle up it with about 10 bar in the compressor tank, the crud will end up in the fuel probably, but that should either disolve into what it was in the first place or get trapped by the fuel tap filters.
CLEM

 

[Paul Marx]

Pressurised blow job.
Nothing like it.

Paul

 

[CLEMTOG]

remind me, its been a long time
CLEM